In article ,
Roger Mills writes:
I have an illuminated bathroom mirror which has two unsilvered vertical
strips with 15w T8 fluorescent tubes behind them. The tubes are powered
via a single 2-way electronic ballast.

The lights have suddenly developed a fault. When turned on, they light
normally but then go out after a few (4 or 5) seconds. If I then turn
them off and wait a minute or so, I can turn them on again - and the
cycle repeats. There's no way they'll stay on more than a few seconds.

Anyone come across this sort of thing? AIUI, the ballast is supposed to
deliver a high start-up current and then cut it to a lower level for
running. So could it be cutting the current to zero rather than to the
proper value?

Is replacing the ballast likely to fix the problem, or could the tubes
be at fault?

The ballast is a Kengo KEB-215. I can't find one of these anywhere, or
even one of another make with the same spec. The nearest I can find is a
Tridonic 22185216 - which is 2 x 18w rather than 2 x 15w. It's also
slightly longer, so I'd have to bodge the mounting brackets. Assuming I
can make it physically fit, is the difference in electrical spec likely
to be a problem?

If it's a good electronic ballast, it will be detecting unbalanced
current flow in the tubes, and shutting down in that case. This would
be because one of the tubes has run out filament emission coating,
and starts operating as a rectifier. If allowed to continue operating,
that end of the tube will overheat, and would likely damage the lampholder
and anything else near the tube end.

If this is the cause, you need new tubes. Often you will see at least
one tube end is blackened too (that's the sputtered off emission coating).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]